First, it is important if the external tool stores the data separately, or needs SoSci Survey to store the data. For my answer, I assume that respondents can access the external user interface (tool) via URL.
In the first case (data stored separately), you can redirect to the URL of the external tool and provide the case number and the case token in the URL, e.g.:
The external tool must then store the case number, and if the questionnaire shall resume later, it must also store the token to redirect the respondent to an address like
If it not necessary to resume the questionnaire, then the initial redirect would look a bit different.
You may also consider opening the external tool in a pop-up window, for example using this HTML code:
<a href="https://MyExternalTool.xyz?id=%caseNumber%" onclick="window.open(https://MyExternalTool.xyz?id=%caseNumber%); return false;">Click here</a>
In the second case (data must be stored by SoSci Survey), use the first redirect, and return the collected data with the back-link to the questionnaire:
In SoSci Survey then use a question "device and request variables" to read the variable "data" into the data set.